1. Field of the Invention
The present invention relates to a blasting method and a blasting machine used for the blasting method and, more particularly, to a blasting method and a blasting machine in which an abrasive containing a certain amount of water or other liquid is used to confer desired elasticity (such abrasive is referred to as “elastic abrasive” in the specification).
2. Description of the Related Art
Compared with polishing using an abrasive paper, an abrasive cloth, a grindstone or the like, blasting in which an abrasive is ejected onto the surface of a workpiece along with a compressed gas, such as compressed air, can be applied relatively easily to an intricately shaped object. Therefore, blasting is widely used for various purposes, including cleaning of mold surfaces, removal of oxide films, and deburring of various products.
For such blasting, sands or abrasive grains are generally used as the abrasive. Therefore, when the abrasive is bombarded on the surface of a product, indentations are formed in the surface. These indentations in the surface of the processed product lead to a rough, satin-like finish similar to the surface of a pear.
For the reason, blasting cannot be employed for products whose processed surface needs to be a smooth or mirror-like surface. If blasting should be employed for such products, additional processing such as deburring needs to be carried out, followed by further lapping or buffing to make the product surface smooth by removing irregularities formed by blasting.
In the manner, blasting has an advantage in that it can be applied to workpieces having various shapes, in other words, can be applied even to intricately shaped workpieces, relatively easily. On the other hand, blasting has a disadvantage in that it processes the surface of a workpiece to a satin-like finish if a general abrasive is used, and therefore, it cannot be employed for applications or materials in which the formation of a satin-like finish is not desired.
To overcome the disadvantage, a blasting method for projecting an elastic abrasive at an incident angle oblique to the surface of a workpiece to provide a mirror-like finish or a smooth surface without forming irregularities on the surface of the processed workpiece has also been proposed. According to the method, the abrasive itself is deformed when it is bombarded on the surface of the workpiece due to the elasticity of the abrasive, and no indentations (irregularities) are formed. In addition, since the elastic abrasive slides on the surface of the workpiece after it has been bombarded on the surface, the workpiece surface can be processed to a flat surface or a mirror-like finish.
A proposed elastic abrasive used for such a blasting method is formed by carrying on abrasive powders (abrasive grains) to the surface of an elastic porous carrier made of, for example, natural vegetable fibers (Japanese Unexamined Patent Publication No. HEI 9-314468). If a carrier formed of vegetable fibers contains water, it polishes the surface of the workpiece almost to a mirror-like finish. However, heat generated during polishing causes water in the carrier to evaporate, thereby decreasing the viscosity and elasticity of the carrier. As a result, the workpiece is processed to a satin-like finish. It should also be pointed out that a recovery rate of the abrasive decreases because of, for example, crushing of the carrier. To prevent the decrease in the recovery rate, blasting with the elastic abrasive formed by adhering abrasive grains onto a carrier made of gelatin containing an evaporation-preventing agent by an adhesive force of the water contained in the carrier has also been proposed (Japanese Patent No. 3376334).
Furthermore, although optimal polishing is ensured while water is retained in the abrasive, water evaporation cannot be completely prevented because the abrasive is continuously used. Although a water-evaporation-preventing agent is contained in the abrasive, a certain degree of water evaporation still occurs. To overcome the problem, an abrasive machine 10 as shown in FIG. 7 has been proposed. In the abrasive machine 10, the abrasive accumulated at the bottom of a cabinet 2 is transported on a conveyor belt 97 up to an opening 85a of an ejecting rotor 85 rotating at the top of the cabinet 2, the abrasive is projected onto a workpiece W also disposed in the cabinet 2 by a centrifugal force along with the rotation of the ejecting rotor 85, and the projected abrasive accumulates at the bottom of the cabinet 2 so that it can be recovered. Furthermore, the abrasive machine 10 includes water-supply means 7′ for spraying water onto the abrasive accumulating at the bottom of the cabinet 2 to maintain the water content of the abrasive (Japanese Unexamined Patent Publication No. 2003-211359).
The above-mentioned '359 application also describes that a test piece is subjected to preparatory polishing without supplying water to measure a decrease in the volume of water in the abrasive over a certain period of time so that water is additionally supplied based on the result of the measurement.
Physical properties, such as the hardness and the elasticity, of the above-described elastic abrasive greatly change depending on the water content. More specifically, the elastic abrasive exhibits desired elasticity when a sufficient volume of water is contained, but becomes harder and more brittle as the water content decreases.
As described above, when water evaporates from the elastic abrasive and the hardness of the elastic abrasive is increased, it becomes more difficult for the abrasive to slide on the surface of the workpiece, or the abrasive bounces off, rather than sliding on the workpiece surface. It causes the surface of the workpiece to become a satin-like finish, which greatly affects the glossiness and the workability of the surface of the processed workpiece. In the case, the elastic abrasive becomes more brittle and the recovery rate of the abrasive decreases.
On the other hand, for the invention disclosed in the above-described '359 application, water is sprayed to the abrasive accumulated at the bottom of the cabinet 2 by using the water-supply means 7′ to replenish the dry abrasive with water. Thus, the invention in the above-described '359 application can be expected to have a certain level of effectiveness against a change in the workability or a decrease in a recovery rate resulting from curing of the abrasive resulting from drying of the abrasive.
It should be noted, however, that the abrasive accumulated at the bottom of the cabinet 2 is large in amount and many layers of the abrasive are deposited one on another in a lump. Therefore, the abrasive may remain in a lump even though the abrasive is stirred somewhat while it is being transported upward by means of the conveyor belt 97. Thus, water sprayed from above onto this solidified abrasive cannot satisfactorily reach the lower layers of the abrasive though such water may reach the upper layers of the abrasive.
What is worse, if a large volume of water is supplied to sufficiently replenish the lower layers of the abrasive with water, the upper layers of the abrasive will be saturated with water. If such abrasive saturated with water is ejected onto a workpiece, the workpiece will be contaminated with water adhering to it. In addition, for blasting machines in which the abrasive is ejected together with a compressed gas, the abrasive in the form of particles aggregated due to water, resulting in relatively large secondary particles, will cause clogging in a blasting gun for ejecting the abrasive.
Furthermore, since the abrasive is ejected onto the workpiece in a state where flexible, a highly elastic abrasive replenished with a sufficient volume of water is mixed with dry, hard, and a brittle abrasive failing to absorb a sufficient volume of water, each component of the abrasive is bombarded on the workpiece surface in different states. Thereby, an expected processing state cannot be achieved, and therefore, the quality of processed products decreases and the defective fraction increases.
In order to supply an optimal volume of water, the invention in the above-described '359 application also discloses the process of measuring a decrease in the volume of water during preparatory polishing to determine the volume of additional water based on the obtained data for the decrease in volume of water.
According to the method, the volume of evaporating water needs to be determined by, for example, obtaining all amounts of the abrasives in the cabinet 2 before and after preparatory polishing and then measuring a change in weight between before and after the preparatory polishing. Not only is the work extremely complicated but also machine operation needs to be suspended during the measurement.
Furthermore, although it is always expected that an optimal amount of water is supplied according to the volume of evaporating water obtained through preparatory polishing, the volume of water evaporating from the abrasive differs at different times depending on various conditions, such as ejection conditions of the abrasive, the shape of the workpiece, and the temperature. In particular, for blasting in which the abrasive is ejected in a state of a mixed fluid containing a compressed gas, such as air, argon, or nitrogen, the volume of evaporating water also changes depending on the components, pressure, temperature, and so forth of the compressed gas with which the abrasive is ejected.
For the reason, in order to allow many processed objects (i.e., workpieces) of the same material and the same shape to always have processing results of constant quality, the water supply amount needs to be adjusted as appropriate according to changes in various conditions during blasting.
However, in the method disclosed in the above-described '359 application, there is no way for acquiring data required for water volume adjustment during blasting. In other words, an excessive or insufficient volume of water may be supplied, which increases the defective fraction.
For the elastic abrasive described in the two other documents mentioned above, the carriers of the abrasive grains are formed of a porous member of natural vegetable fibers or gelatin, and the abrasive grains are carried on the surfaces of the carriers by an adhesive force of water contained in these carriers. Consequently, when the adhesive force is lost due to drying, some abrasive grains fall out, and therefore, the cutting force decreases.
Furthermore, even in a state where a sufficient volume of water is contained, once the abrasive carried on the surfaces falls out, the cutting force cannot be recovered. Moreover, when burrs or cut powders generated during processing adhere to the abrasive surface by the adhesive force of the carriers, they add to the cutting force of the abrasive grains. For the reason, the surface of the workpiece may be cut more than expected, failing to produce a mirror-like finish, in other words, failing to process the workpiece to have a desired surface.
In view of these circumstances, the present invention has been conceived in order to overcome the problems associated with the above-described related art. More specifically, in a blasting method using the elastic abrasive that contains a certain amount of liquid to exhibit desired elasticity, the liquid content of the individual abrasive to be ejected onto a workpiece is maintained uniform and constant. Thereby, an object of the present invention is to provide a blasting method and a blasting machine for realizing the method in which variations in quality can be prevented, and consequently, the defective fraction can be reduced even when many workpieces are sequentially processed.
Another object of the present invention is to provide a blasting method and a blasting machine capable of adjusting the volume of supplied liquid according to a change in evaporation rate of the liquid impregnated into the abrasive resulting from a change in processing conditions, thereby supplying an optimal amount of liquid.
Still another object of the present invention is to provide a blasting method capable of processing a workpiece with desired high processing accuracy by the use of the elastic abrasive which can prevent the cutting force from changing for various reasons, such as abrasive grains falling off or adhesion of burrs and cut powders generated during blasting to the workpiece surface.